WASHINGTON - The Pentagon this week approved a major new policy directive that elevates the military's mission of "irregular warfare" - the increasingly prevalent campaigns to battle insurgents and terrorists, often with foreign partners and sometimes clandestinely - to an equal footing with traditional combat.

The directive, signed Monday by Deputy Defense Secretary Gordon England, requires the Pentagon to step up its capabilities across the board to fight unconventionally, such as by working with foreign security forces, surrogates, and indigenous resistance movements to shore up fragile states, extend the reach of U.S. forces into denied areas, or battle hostile regimes.

The result of more than a year of debate in the defense establishment, the policy is part of an overhaul of the U.S. military's role as the threat of large-scale combat against other nations' armies has waned and new dangers have arisen from shadowy non-state actors, such as terrorists that target civilian populations.

"The U.S. has considerable overmatch in traditional capabilities . . . and more and more adversaries have realized it's better to take us on in an asymmetric fashion," said a chief architect of the policy, Michael Vickers, assistant defense secretary for special operations/low-intensity conflict and interdependent capabilities.

Designed to institutionalize lessons the U.S. military has learned - often painfully - in Afghanistan and Iraq since 2001, the policy aims to prepare the military for the most likely future conflicts and to prevent the type of mistakes made in the post-Vietnam era, when hard-won skills in counterinsurgency atrophied.

Defense Secretary Robert Gates has lobbied outspokenly for such a shift.

In September, he said: "Think of where our forces have been sent and have been engaged over the last 40-plus years: Vietnam, Lebanon, Grenada, Panama, Somalia, Haiti, Bosnia, Kosovo, Afghanistan, Iraq, the Horn of Africa, and more. In fact, the first Gulf War stands alone in over two generations of constant military engagement as a more or less traditional conventional conflict."

Gates warned that, for the foreseeable future, the United States would face the greatest threats from insurgents and extremist groups operating in weak or failing states. "We do not have the luxury of opting out because they do not conform to preferred notions of the American way of war," he said.

The 12-page directive states that irregular warfare "is as strategically important as traditional warfare."

Defined as "a violent struggle among state and non-state actors for legitimacy and influence over the relevant population(s)," irregular warfare "favors indirect and asymmetric approaches to erode an adversary's power, influence, and will," the directive states.

A major thrust of the policy is for U.S. troops to do less of the fighting and instead build the capabilities of foreign militaries and security forces. The goal, Vickers said in a recent speech, is "to create a persistent, ubiquitous presence against our adversaries . . . and essentially to smother them over time."